Editing Inflaton (section)

== Cosmological inflation ==
{{main|Inflation (cosmology)}}

The basic{{clarify|date=October 2016}} model of inflation proceeds in three phases:<ref>{{cite arXiv|author=Tsujikawa, Shinji|title=Introductory review of cosmic inflation|year=2003|eprint=hep-ph/0304257}}</ref>
* Expanding vacuum state with high potential energy
* [[Phase transition]] to true vacuum
* Slow roll and reheating

=== Expanding vacuum state with high potential energy ===

A ''"vacuum"'' or ''"[[vacuum state]]"'' in [[quantum field theory]] is a state of quantum fields that is at [[local minimum|locally minimal]] potential energy. Quantum particles are excitations that deviate from this minimal potential energy state, therefore a vacuum state has no particles in it. Depending on the specifics of a quantum field theory, it can have more than one vacuum state. Different vacua, despite all "being empty" (having no particles), will generally have different [[vacuum energy]]. Quantum field theory stipulates that the pressure of the vacuum energy is always negative and equal in magnitude to its energy density.

Inflationary theory postulates that there is a vacuum state with very large vacuum energy, caused by a non-zero vacuum expectation value of the inflaton field. Any region of space in this state will rapidly expand. Even if initially it is not empty (contains some particles), very rapid exponential expansion dilutes any particles that might have previously been present to essentially zero density.

=== Phase transition to true vacuum ===
{{Unreferenced section|date=August 2019}}
Inflationary theory further postulates that this "inflationary vacuum" state is not the state with globally lowest energy; rather, it is a "[[false vacuum]]", also known as a ''[[metastable]]'' state.

For each observer at any chosen point of space, the false vacuum eventually tunnels into a state with the same potential energy, but which is not a vacuum (it is not at a local minimum of the potential energy—it can "decay"). This state can be seen as a true vacuum, filled with a large number of inflaton particles. However, the rate of expansion of the true vacuum does not change at that moment: Only its exponential character changes to much slower expansion of the [[FLRW metric]]. This ensures that expansion rate precisely matches the energy density.

=== Slow roll and reheating ===

In the true vacuum, inflaton particles decay, eventually giving rise to the observed Standard Model particles.{{cn|date=June 2022}} The shape of the potential energy function near "tunnel exit" from false vacuum state must have a shallow slope, otherwise particle production would be confined to the boundary of expanding true vacuum bubble, which contradicts observation (the universe we see around us is not built of huge ''completely'' void bubbles). In other words, the quantum state should "roll to the bottom slowly".

When complete, the decay of inflaton particles fills the space with hot and dense Big Bang plasma.<ref>{{cite web |first=Matt |last=Strassler |date=17 March 2014 |title=Inflation |department=History of the Universe |series=Relativity, space, astronomy, and cosmology |website=Of Particular Significance (profmattstrassler.com) |type=blog |url=https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/inflation/ |access-date=2024-08-03 |lang=en-US |quote=... by the author of ''Waves in an Impossible Sea''. |df=dmy-all }}</ref>